We have performed direct observations on the conformational change of a single double-stranded T4 DNA molecule induced by diaminoalkanes, NH3+(CH2)nNH3+(n = 1-6), by use of fluorescence microscopy. It was found that diamines with three and five methylene groups show a significant effect on the compaction of individual single DNA molecules. On the other hand, diamines with two, four and six methylene groups are not effective in the compaction of DNA. Such a drastic difference in the action of diamines suggests that not only the number of charge but also the length between the amino groups in the diamines plays an essential role in their interactions with DNA.